The invention relates to an apparatus for separating water-containing solvent mixtures having two or more phases, with a gravitational separator, and to the use of the apparatus, and also to a process for separating water-containing solvent mixtures having two or more phases.
In a variety of industrial cleaning processes, mixtures occur of polar (water) and non-polar solvents (organic solvents) and require separation. Textile-cleaning plants for dry cleaning are an example of this. They operate predominantly with non-polar organic solvents, such as halogenated or non-halogenated hydrocarbons, e.g. tetrachloroethane (perc) or isoparaffinic hydrocarbons (HCS).
When the cleaned textiles are dried, the solvent is evaporated in a stream of hot air and then condensed out in a condenser. During this, water is entrained alongside the organic solvent and is a second phase alongside the organic solvent in the condensate. To allow reuse of the solvent for dry cleaning textiles, the aqueous phase has to be removed.
This also applies when distillation is used to treat the soiled organic solvent used for textile cleaning. Here again, before the organic solvent is reused the aqueous phase has to be removed.
Finally, this also applies when distillation is used in sectors other than dry cleaning to treat soiled organic solvents. Here again, after distillation and prior to reuse of the organic solvent the aqueous phase frequently has to be removed.
The water is usually removed in gravitational separators. The difference in the density of organic solvent and water, which are practically insoluble in one another, results in separation of the water from the organic solvent in the separator. If the density of the water is higher, it settles at the base, and if it is lower it floats on top of the solvent, and in both cases can be discharged. This procedure can be carried out effectively and without difficulty for the commonly used dry cleaning solvents, such as tetrachloroethene (perc) with a density of 1620 kg/m3 (at 20œ C.) and hydrocarbon solvents (HCS) with a density of about 780 kg/m3 (at 15œ C.).
One of the reasons why the use of other solvents (e.g. aliphatic glycol ethers) suitable for removing specific types of soiling fails is that the density of these is very little different (less than 100 kg/m3) from that of water, so that in the prior art relating to water separators no reliable process is available for the required separation of the condensed organic solvent from the water. It has therefore hitherto been impossible to use these types of solvent, which could give great advantages in the cleaning of certain types of product.
The object of the invention was therefore to provide an apparatus for separating water-containing solvent mixtures having two or more phases, e.g. for textile cleaning plants, and capable of effectively removing water, even when the density difference from the organic solvent is small. For the purposes of the present invention, a small density difference is in particular a difference of less than 100 kg/m3.
This object is achieved by an apparatus with the features of FIG. 1.
By virtue of the fact that there is a connection between a feed line of the gravitational separator and the outlet of at least one coalescence separator, and that therefore a coalescence separator has been provided upstream of the gravitational separator, relatively large droplets of water or, respectively, of organic solvent are first produced in the condensate resulting from the reclaim of the solvent used for textile cleaning. If the solvent mixture, which by this stage comprises droplets (normally of water) which are substantially larger than in the condensate immediately after its deposition, is fed to a gravitational separator it has been found that, even when the density differences are small, the separation rate achieved is then sufficient for industrial application. On the other hand, the increase in the separation rate permits smaller dimensioning of the gravitational separator.
For the purposes of the present invention, coalescence is the complete merging of bubbles or drops in a fluid once they have approached one another (cf. xe2x80x9cABC Verfahrenstechnikxe2x80x9d [The A to Z of Process Engineering], VEB Verlag Leipzig, 1979). A precondition for this complete merging is that the particles contact one another and that the layer situated between the particles break apart.
A suitable method for increasing the droplet size has been found to be charging the coalescence separator with an open-pore solvent-resistant foam, for example. Using a polyurethane/polyester foam material has proven particularly suitable. The pore size of the foam here is preferably from 440 to 520 (m.
A further improvement in the separation of aqueous and organic phase is achieved if a circulation line has been provided between gravitational separator and coalescence separator. Via this circulation line, that proportion of the solvent mixture in which the separation into aqueous and organic solvent is not yet sufficient can be fed back to the coalescence separator, where enlargement of the droplet size again occurs. The inlet to the circulation line is preferably in a central region of the gravitational separator in which incompletely separated mixture is present during operation.
It is advantageous for the amount of the solvent mixture fed back again into the coalescence separator to be controlled so that no water, or only a small amount of water, emerges at the exit from the gravitational separator for the organic solvent. To this end, a sensor may have been provided in the gravitational separator, e.g. based on conductivity measurement or opacity measurement. The arrangement of this sensor is such that a signal is triggered if water passes close to or into the exit of the gravitational separator. This signal causes the opening of a valve, whereupon that portion of the solvent mixture in which separation into aqueous and organic phase is not yet sufficient is fed again to the coalescence separator, e.g. with the aid of a pump.
The feed of the solvent mixture from the coalescence separator into the gravitational separator can produce turbulence in the gravitational separator. To achieve the highest possible purity of solvent passing out of the gravitational separator it is intended that the feed line provided between the coalescence separator and the gravitational separator should open into the gravitational separator at a distance vertically from the surface of the solvent and at a distance vertically from the base of the gravitational separator. By increasing the height of fill (=designed height) of the gravitational separator it is possible to increase the residence time of the solvent/water mixture, and this is particularly advantageous if the difference in density between the polar and non-polar phase is very small.
If the difference in density between the organic solvent and the water is very small, it is also advantageous for a second gravitational separator to be used to increase the fill height and thus prolong the residence time. The required rise in fill height can be achieved using a second feed line, provided between the first and second gravitational separators and arranged so that one solvent phase, preferably the organic solvent phase, is fed from the first gravitational separator to the second gravitational separator. The feed may in particular use the pipe-link principle.
Turbulence is produced in the first gravitational separator by the feed of the solvent mixture from the coalescence separator and the circulation line optionally provided from the first gravitational separator to the coalescence separator. This, together with the surfactants and other surface-active substances present in the solvent mixture makes it possible that when the density difference between aqueous and organic solvent is very small there can still be water present in the organic solvent discharged from the gravitational separator. It is therefore advantageous for a second gravitational separator to have been provided, in which further phase separation can occur. Attention should be paid here to reduce turbulence as far as possible, and also preferably to keeping the flow rate low. It is therefore particularly advantageous for the connection between the first and second gravitational separators to use the pipe-link principle. The separated aqueous phase is refed to the first gravitational separator. As an alternative to the installation of a second gravitational separator, the design height of the first gravitational separator may be increased appropriately if space is available.
The novel apparatus is particularly suitable for separating solvent mixtures from textile cleaning. The use of the apparatus is particularly advantageous if the solvent mixture has been formed from water and an organic solvent of comparable density, in particularly propylene glycol ether (density 960 kg/m3) or a polydimethylsiloxane (density about 960 kg/m3).
A further advantage is achieved if the higher-density phase collected in the lower part of the gravitational separator and separated off is discharged automatically. This phase may in particular be water. This objective is achieved by using a sensor in the lower part of the gravitational separator to establish whether the water has separated from the organic solvent. If this is the case, the sensor opens the run-off valve, activating discharge of the water collected. The operation of the sensor may be based on conductivity measurement or opacity measurement, for example.
A further object of the invention was to provide a process which can separate a water-containing solvent mixture having two or more phases, where the constituents of the mixture have comparable density.
The novel process comprises the steps:
feeding a condensed-out water-containing solvent mixture containing two or more phases to a coalescence separator;
enlarging the volume of the droplets, preferably of the water droplets, in the coalescence separator;
feeding the solvent mixture with enlarged droplet volume to a gravitational separator;
separating the solvent mixture in the gravitational separator into a first phase which is composed predominantly or exclusively of organic solvent (phase A), into a second phase, which comprises a mixture of organic solvent and water (phase B), and also into a third phase, which is composed predominantly of water (phase C).
If the density difference is very small it is advantageous for the second phase to be circulated between the gravitational separator and coalescence separator, preferably with control via a sensor. The sensor detects, for example, the water content of the phase present. Improved phase separation can also be achieved by increasing the fill height of the gravitational separator.
The water content in the organic solvent can be further reduced if the first phase is fed to a second gravitational separator, preferably at a low flow rate and with low turbulence, and in particular using the pipe-link principle. Further separation of the solvents takes place in the second gravitational separator.